1. Field of the Invention
The present invention relates to a structure of a thin film transistor (TFT) array and a method for fabricating the same. More particularly, the present invention relates to a structure of a TFT array and a method for fabricating the same by using four masks.
2. Description of Related Art
As going with the development of the flat panel display, the cost of thin film transistor liquid crystal displays (TFT LCDs) has been continuously reduced and the market share of TFT LCD has continuously increased. Therefore, how to reduce the fabrication cost is the most concerned issue for manufacturers. The fabrication of the current TFT array of a TFT LCD requires five masking processes, i.e., five photolithography processes. Each masking process includes steps of coating the photoresist, exposing and developing the exposed photoresist and removing the photoresist. Therefore, if one of the photolithography processes can be saved, the fabricating cost can be greatly reduced and the throughput can be significantly increased. Moreover, the larger the panel substrate is, the more expensive the mask will be. Therefore, saving one mask can significantly reduce the fabrication cost.
FIG. 1 shows a cross-sectional view of the existing TFT array fabricated by using five masks. First, a metal layer is formed on the substrate 100 and then patterned using the first mask to form a gate electrode 102 and a storage capacitance electrode 104. Then, a gate insulating layer (silicon nitride) 106, an amorphous silicon layer 108, and an N+ implanting layer 110 are deposited on the substrate 100. The second mask is then used to pattern the amorphous silicon layer 108 and the N+ implanting layer 110. The deposition of an ITO (indium tin oxide) transparent electrode is performed and an ITO transparent electrode 114 as shown in FIG. 1 is defined by the third mask. A source/drain pattern 112 is defined by the fourth mask. A passivation layer is subsequently deposited, and the passivation layer 116 is defined by the fifth mask. Accordingly, for fabricating the conventional TFT structure, five masks are inevitably required, which would increase the fabrication cost.
Several methods have been proposed to reduce the number of masks, among which a half tone or gray tone mask is used to replace two aforementioned masks. After the half tone exposure and development, a gate area is etched, and then a source/drain region is etched. However, the half tone or gray tone mask is very expensive. Though the number of masks may be reduced, an additional a photoresist etching process is required. In this way, the fabricating cost cannot be significantly reduced. Moreover, it is hard to control the pattern when a half tone or gray tone mask is employed, causing dimension error and other problems. Thus, the fabrication yield is lowered.
FIGS. 2A to 2D are cross-sectional views of another process for fabricating a TFT array using four masks disclosed by U.S. Pat. No. 6,891,196. As shown in FIG. 2A, a metal layer, an insulating layer and a silicon layer are formed on the substrate 201. An island region having a gate electrode 202, a gate insulating layer 203 and a silicon layer 204 is then defined by a mask. Next, a passivation layer 205 is formed and a source/drain 206 is defined. A passivation layer 207 is formed, and the passivation layers 205, 207 are patterned using a mask. Next, an ITO electrode layer 208 is formed, and then patterned using a mask. According to this method, a portion of the ITO layer 208 is used as an ordinary pixel electrode, and another portion thereof is used as a contact electrode of the source/drain. However, the method employs a special process to perform amorphous silicon implantation, i.e., using high-temperature plasma to diffuse phosphor, so as to form an N+ implanting layer. Therefore, though the number of masks is reduced, a special process is required to form the N+ implanting layer and the TFT structure also becomes complicated.
As a result, how to reduce the total number of masks and how to use an ordinary process to reduce the cost have become an important issue to the manufacturers in the field.